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Evaluation of DTP scannersa case study with Agfa Horizon

Author(s): Baltsavias, Emmanuel P.

Publication Date: 1993

Permanent Link: https://doi.org/10.3929/ethz-a-004336934

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Evaluation of DTP Scanners - a Case Study with Agfa Horizon

Emmanuel P. Baltsavias

Institute of Geodesy and Photogrammetry

Swiss Federal Institute of Technology

ETH-Hoenggerberg, CH-8093 Zurich, Switzerland

Tel.: +41-1-6333042, Fax: +41-1-3720438, e-mail: [email protected]

ABSTRACT

Scanners are an essential part of softcopy photogrammetric systems. Although thedevelopments in direct digital data acquisition have been enormous in the last decade, film-based systems are used in all fields of photogrammetry. The main use of scanners today isdefinitely in the digitisation of aerial images, particularly for digital ortho-image and DTMgeneration, and integration of image data in GIS. So called photogrammetric scanners are inmost cases very expensive, while at the same time they may exhibit significant errors,particularly in radiometry. DTP scanners have low price, and may have a good performanceand sufficient resolution. Their main disadvantage is the lack of geometric accuracy andstability. By using calibration techniques, the geometric errors can be kept to less than 0.5pixel of the highest geometric resolution, making thus such scanners suitable for mostphotogrammetric applications. This paper presents a short overview of scanners, adescription of the 1200 dpi scanner Agfa Horizon, and finally test procedures and results ofthe radiometric and geometric quality of the scanner.

1. INTRODUCTION

Scanners are an essential part of softcopy photogrammetric systems. Although thedevelopments in direct digital data acquisition have been enormous in the last decade, film-based systems are used in all fields of photogrammetry. In close-range applications manymetric, semi-metric and amateur cameras still use films. The reasons are the high filmresolution, and low costs, ease-of-use and availability of amateur and semi-metric cameras.In aerial photogrammetry film-based systems will provide the main data input for manyyears to come. Film-based satellite images are provided by many Russian sensors, while alsoin USA companies, previously active in military applications, want to launch film-basedhigh resolution commercial systems. The main use of scanners today is definitely in thedigitisation of aerial images. The main applications that increase the need for digital aerialdata are (i) the digital ortho-image generation, (ii) the automatic DTM generation, and (iii)the integration of digital data, particularly ortho-images and derived products, in GIS.

The aim of this paper is to look into more detail into DTP scanners, and especially the AgfaHorizon, and present their major characteristics and problems. The here presented work will

also include preliminary studies on specific important questions, like: how good are DTPscanners for photogrammetric tasks?, can their errors be corrected by calibration procedures?,at what costs?, what accuracy can be achieved after calibration?

1.1. SCANNER CLASSIFICATION

Scanners of documents (reflective and/or transparent) can be classified according to theirfunction in the following categories:

1. photogrammetric scanners

2. modified analytical plotters or monocomparators

3. scanners of large documents

4. microdensitometers

5. DeskTop Publishing (DTP) scanners

6. scanners of documents and 3-D objects

7. slide scanners

8. document scanners (with OCR software)

9. multiple purpose scanners (scanner/copier/colour printer, similar to DTP scanners butmore expensive and with a typical resolution of 400 dpi ; scan/edit/fax scanners)

10. other specialised scanners (hand-held scanners, engineering document scanners,roentgen-image scanners, microfiche digitisers, barcode scanners)

Assuming that for photogrammetric applications, a scanner should be able to scan aerialimages (23 x 23 cm) with a minimum optical resolution of 600 dpi and sufficient radiometricand geometric accuracy, only scanners of the first five categories should be addressed.

Photogrammetric scanners are produced by companies involved in photogrammetry,have a high geometric accuracy (typically 2 - 4 µm), sometimes software for interiororientation and epipolar transformation, but the majority of them are very expensive(300,000 - 400,000 SFr.). In the opinion of the author the most expensive photogrammetricscanners are sold at excessive prices, while there is a convergence between the lower pricedones and the top DTP scanners (see below). Expensive photogrammetric scanners mayexhibit considerable errors particularly in radiometry. Figure 1 shows a part of an image ofthe OEEPE test on digital aerial triangulation, stretched with a gamma of 3. The imageswere scanned with a Zeiss/Intergraph PS1 using a 2048 element linear CCD which scans theimage in swaths. As it is seen at the center of Figure 1 the border line between neighbouringswaths is clearly visible. The grey level differences between neighbouring pixels on eitherside of the line in the original image were up to 55 grey values. The pixels that had greyvalue 0 are displayed in white. As it can be seen, the image is completely saturated and thesignal is lost. This may be due to various reasons: wrong setting (automatic or manual) ofthe density range during scanning, insufficient dynamic range of the CCD, wrong exposure

of the film, drop-off of the illumination at the border of the scan area (notice the film borderat the right of Figure 1 ; it is not however saturated, so this reason is not very probable).Figure 2 shows another example of the same problem, and additionally vertical dark stripeswhich must be either due to defective sensor elements or wrong radiometric calibration ofthe individual sensor elements (see also Colomer, 1993). Geometric errors may also occurwith photogrammetric scanners. Images scanned with the Leica-Helawa DSW 100 for ourInstitute exhibited misregistration errors of the individual CCD frames from which theimage consists of more than 5 pixels, while in other cases no error was observed. Puttingtogether the individual CCD frames in a correct radiometric and geometric way is also aproblem for the Signum HIRES scanner.

Figure 1. Part of an image of the OEEPE test on digital aerial triangulation digitised with

b)

a)

a Zeiss/Intergraph PS 1; a) radiometric differences between neighbouringswaths scanned by the linear CCD, b) pixels displayed in white have grey value0 (saturation).

Figure 2. Same data set as in Figure 1. Vertical dark stripes are visible at positions a) andb).

Scanning by using analytical plotters equipped with CCD cameras never becamepopular, and it is very unlike that this will change. Analytical plotters are widespread, have ahigh geometric accuracy, and are equipped with photogrammetric software. However,installation of CCDs at analytical plotters require modifications, that are very oftenexpensive, the costs of the hardware (cameras, framegrabbers, monitors) and the necessary

a)

b)

calibration software is not inexpensive either, the scanning process is slow, and theradiometric quality suffers because illumination, optics, filters etc. are not optimised forscanning. Additionally, the concept of a scanner does not fit very well to analytical plotterswhich, by their very nature, are operator driven, and apart from that scanning would obstructthe normal work at the analytical plotter.

Scanners of large documents are in most cases drum scanners, and thus their geometricaccuracy is generally less than that of flatbed scanners. They may have a high radiometricresolution, but often they do not scan transparencies. Often they can also plot, and thus arequite expensive. Their main application is in scanning of large format plans and maps, andless in scanning of films.

Conventional microdensitometers (Perkin-Elmer, Joyce-Loebl) have a very high geometricresolution (up to 2 µm), a very good geometric accuracy and radiometric quality (12 - 16 bit)but they are not very widespread, are generally very expensive, slow, quite complicated tocalibrate, and less flexible both hardware- and software-wise. They are important for analysisof micro-image properties and specific applications where radiometric resolution and accuracyare of major concern (astronomy, medicine, image quality determination).

DTP scanners have been developed for applications totally different than thephotogrammetric ones. However, since they constitute the largest sector in the scanner market,they are subject to rapid developments and improvements. The consultancy BIS StrategicDecisions (Norwell, MA) forecasts that the colour flatbed DTP scanner market will grow 39 %annually over the next five years (Holch, 1993). DTP scanners are usually flatbed employinglinear CCD sensors, but there are also a few drum scanners using photodiodes or PMTs. Theirgeometric resolution is constantly increasing and today there are scanners with 1200 dpiresolution (this refers to flatbed scanners, drum scanners can have a resolution of up to 4,000dpi), capable of scanning aerial image formats in both reflective and transparency modes, in B/W or colour. Their radiometric resolution and quality, and scanning speed can be comparableor even exceed that of the much more expensive photogrammetric scanners. They aresupported by many computer platforms and sophisticated software for setting the scanningparameters, image processing and editing, and colour editing. Their price is generally in the1,000 - 50,000 SFr. range, whereby scanners with a true (optical) resolution of 1200 dpi, andespecially with a scanning format larger than A4 are towards the high end of the price range.Scanners with a 11 by 17 inches format, a resolution of 600-1200 dpi, and in most casestransparency options are offered by Agfa (Horizon, ACS-100), Howtek (Scanmaster), Pixel-Craft, Sharp (JX-600), Imapro (QCS-2400), Truvel (TZ-3BWC), ANA Tech (Eagle 1760), andScitex (Smart). For more details on DTP scanners see Holch, 1993. Good drum scanners (theScanMate magic of the danish firm ScanView and the Howtek Scanmaster D4000) with aresolution of 2,000 - 4,000 dpi, and A4 or 25 x 25 cm format cost 25,000 - 80,000 SFr.respectively. The main disadvantage of DTP scanners is the insufficient geometric accuracy,caused mainly by mechanical positioning errors and instabilities, large lens distortions, andlack of geometric calibration software. Further improvements in the previous topics from theside of the manufacturers can not be excluded, and even now a user can develop owncalibration software and by using appropriate test patterns and test procedures, a geometric

accuracy of less than 0.5 pixel can be achieved. For these reasons and because of theirattractive characteristics and low price, the DTP scanners should be analysed more carefully.

Scanners can be classified according to different criteria:

A. Dimensionality of simultaneously sensed elements

• point sensors (one pixel scanned at a time)

They are often used in microdensitometers and drum scanners for high geometric andradiometric resolution. Generally each pixel is illuminated by a laser beam and thesensor consists of a photomultiplier tube (PMT). PMTs have a very high speed andvery high dynamic range. Microdensitometers and some drum scanners have a greatflexibility in selecting different pixel sizes and forms and x-, y-spacing between pix-els. DTP drum scanners use cheaper illumination/sensor systems, e.g. halogen lampsand usually photodiodes (their dynamic range is less than that of PMTs but higherand with a more linear response than that of CCDs).

• line sensors

The line usually consists of CCD elements but also photodiodes or charge-coupledphotodiodes. There might be one linear CCD, or more, whereby in the latter case thelinear CCDs are optically butted using beam splitting prisms or rotating mirrors. Forcolour scanning one or multiple 3-chip linear CCDs may be used.

• area sensors

They consist of CCD chips with a resolution ranging from 512 x 512 to 3000 x 2300pixels.

B. Scanning pattern/movement

• drum scanners

Generally both stage and sensor are moving.

• flatbed scanners

The following cases can be distinguished: moving stage/stationary sensor, stationarystage/moving sensor, both sensor and stage stationary. To achieve high geometric res-olution either the stage or the sensor is moving, whereby the movement can be in twoor one direction. Movement in two directions can be realised by all type of sensors(point, line, area), movement in one direction only by linear CCDs with many sensorelements. The disadvantage of the first case is that it requires high geometric accura-cy in two directions. In addition, in the case of line and area sensors there are oftenclearly visible radiometric differences along the seam lines of neighbouring linestripes or area patches due to illumination instabilities and different sensor elementresponse. The a posteriori correction of this problem requires scanning of neighbour-ing lines/patches with overlap and application of a radiometric equalisation proce-dure as in mosaicking. The documents are usually scanned by a mechanicalmovement, however, electronic deflection of the beam or optical scanning (oscillating

mirrors, rotating prisms) can also be used. The light source either illuminates thewhole object to be scanned or only the portion that it is scanned each time (thus, if thesensor is moving the light source must also move synchronously).

C. Other characteristics

Various other properties like format, resolution, reflective/transparent originals, bina-ry-B/W-colour could be used for a scanner classification.

1.2. SOME SCANNER ASPECTS

1.2.1 Illumination

The illumination must be high in order to achieve a better radiometric quality and higherSNR. The higher the scanning speed, the higher the illumination should be since the dwellingtime (integration time for CCD sensors) is reduced. On the other hand, high power lightsources generate heat, which must be treated appropriately (cooling, use of cold light,placement of the light source away from the sensitive scanner parts), in order to minimise theinfluence on the mechanical parts and the electronics. The spectral properties of the lightsource and its temporal stability (related also to the power supply stability) are importantfactors. Light sources usually include halogen and fluorescent lamps (often over 100 W), aswell as laser beams.

1.2.2 Dynamic range and quantisation bits

The dynamic range of films can be in extreme cases very high (e.g. 16,000:1). To capturethis information a quantisation up to 16-bit would be necessary. Apart frommicrodensitometers, there are photogrammetric, drum and DTP scanners that have A/Dconverters with 10 - 12 bit quantisation, but since almost all software and hardware supportsonly 8-bit/pixel and to avoid problems with excessive amount of data and image display, thedata is reduced to 8-bit. The user often has no influence and no information on how thisreduction is made. If this is done properly, then the result will be a radiometrically better imagewith higher SNR. However, 10 or 12 bit quantisation can lead to an improvement only for lownoise levels. This is not always the case, particularly with CCD based scanners. Parameterslike cooling, maximum charge storage capacity, integration time, smearing etc. are notoptimised to allow a truly beneficial 12-bit quantisation. The 12-bits are used as a sellingargument but often they do not reflect an essential quality difference to 8-bit scanning.

1.2.3 Gamma correction

Often users apply a gamma correction during or after scanning. This is particularly thecase with aerial films that appear relatively dark, especially at the borders. Thus, a gamma of1.5 - 2 is often used to make these regions better visible. Some scanner vendors also useinternally a gamma correction. This processing is appropriate if the image will besubsequently used only for visualisation purposes. Any other use for geometric or semanticinformation extraction will be negatively influenced by the gamma correction, since the darkgrey levels will be stretched, but without occupying a larger number of grey values, and the

bright grey values will be compressed, thus resulting in information loss (or the opposite ifnegative films are used). A better way to improve the contrast is to use off-line after scanninganother algorithm that does not result in less grey values or even better a locally adaptivealgorithm that leads in a larger number of grey values. A local gamma correction isacceptable if it is applied locally to very dark fiducial marks (using ROI processing), in orderto improve their visualisation for subsequent manual or semi-automated measurement.

1.2.4 Colour scanning

Colour scanning can be implemented by:

• primary or complementary colour filters spatially multiplexed on the sensor elements(1-chip colour linear or area CCD)

• use of 3-chip CCDs (linear or area arrays)

• rapidly strobing fluorescent lamps and dichroic filters

• use of a rotating filter wheel before the sensor (RGB and neutral filters) or rotatinglamps

The first three approaches require one scan, while the last one three. The first approachleads to reduced spatial resolution and sometimes pattern noise in the image, and it lacks theability to colour balance (blue in particular). The second is the best approach but also mostexpensive. Although many claim that the third approach is faster than the fourth, the scanningtime is similar, if the same integration time for each colour is required. Another general beliefthat, the third approach often leads to smearing, while the fourth might suffer frommisregistration of the three channels, is also not always correct.

1.2.5 Linear versus area CCDs

Among the sensors, the most promising and widely used are linear CCDs. Today there arevarious linear CCDs with 5,000 to 10,000 elements (the MN3666 from Matsushita with 7,500elements, a 3-chip colour CCD from Kodak with 2,000 - 8,000 elements, TCD 141 fromToshiba with 5,000 elements, the firm Technolink (Americas) sells linear chips with 10,000elements, Thomson has linear arrays with 5184 elements, Dalsa with up to 6,000 elements, andthe list is continuously growing). With current technology multiple linear CCDs can beoptically butted with high precision to result in a line with sufficient elements for a highresolution scan of close to 10 µm. Although area CCDs have a larger number of elements andtheoretically should lead to a faster scanning, they have several disadvantages. Large areaCCDs are very expensive and have blemishes, and standard CCDs need a lot of frames (andtime) to cover the whole image and a precise positioning in two directions as compared to along linear CCD which can scan the whole image in one swath. Radiometric differences alongthe seam lines of the frames can occur as explained above. Linear CCDs provide a betterradiometric quality, need less integration time, have higher charge transfer efficiency, andsuffer less from electronic noise (smearing, blooming etc.) than area CCDs. Linear CCDs havehigher speed (pixel rates of up to 120 MHz) than standard area CCDs, and higher dynamicrange (10,000:1 is possible, compared to 1,000:1 of a very good CCD). Area CCDs aredesigned to maximize the gain (the output signal) which also increases the noise, while linear

CCDs are designed to maximize the dynamic range, so they have less noise and thereforerespond to less input light and due to the higher dynamic range they will respond to higherlight levels as well. Compared to area CCDs they have much lower noise because they havefewer clock signals, so the latter can be better isolated from the video. They have adjustableintegration time while area CCDs are usually locked to the RS170 or CCIR specifications (33or 40 ms respectively). Parallel output for high data rates is easier with linear CCDs. Each linecan be accessed immediately after integration, while with area CCDs all preceding lines mustbe first read out, and they are usually interlaced (so the first even line can be output only afterall odd lines have been read out). Since a line contains a single row of pixels, the uniformitycan be held much tighter than in an area array with several hundred-thousand pixels. They alsohave a more precise geometric centring. In very high precision imaging applications, contrastcorrection hardware and software algorithms are more easily implemented over a single line ofpixels. Antiblooming drains which are important with high contrast objects are easier toimplement with linear CCDs.

Linear CCDs also have some disadvantages. Normal operation of linear CCDs results inmuch shorter integration times than that of area CCDs, and therefore a much greater lightintensity is required. Linear CCDs due to their long length place special demands upon lensesand associated optics. They usually have smaller pixel size than the area CCDs, thus usuallysmaller maximum charge storage capacity. When applying subsampling, linear CCDs have aworse behaviour than area CCDs due to gaps in the scanning direction (see also subsamplingerrors in section 3.1.).

1.2.6 Scanning speed

High or low scanning speed? Many users are fascinated by high speed, and vendors of highspeed scanners use this as a selling argument. First of all, the total time for a successful scanshould be taken into a account. As an example, the Agfa Horizon needs 2 min to scan, transferto swap disk space of the host computer and display in a window, a 30 Mb image. The scannercan mechanically move the 15,000 linear CCDs at up to 100mm/s, which means for a 1200 dpiresolution a grey level image of 15,000 x 4724 pixels could be scanned in 1 sec., i.e. a 71 MHzbandwidth. The electronics of the scanner have however a maximum bandwidth of 15 MHz, sothe maximum scanning speed that can be realised in this case is 21 mm/sec. The samerealisable maximum scanning speed is enforced because of the signal integration time of 1 ms,i.e. in one second 1000 pixels with 21.17 µm pixel size can be scanned. With 15 MHzbandwidth the 30 Mb image can be scanned in 2 sec. The majority of the required 2 min is fortransfer of the data via the SCSI interface (max transfer rate 1.5 Mb/sec) to the host and forsaving the data on disk. That means that the physical scanning process could be much slowerwithout increasing considerably the overall time, i.e. with a 30 times slower scanning rate (0.5MHz) the overall time would be 3 instead of 2 min (actually Horizon permits only a 4 timesslowing scanning rate as the minimum scanning speed is 5 mm/s). This is still not the total timeneeded for a successful scan. The time needed to set and optimise the scanning parameters canbe much more than the time required to do the final scan (it has been reported that a veryexpensive photogrammetric scanner needed 0.5 h for a high resolution scan of an aerial image,but 1 h was needed to set the appropriate scanning parameters, see also Colomer, 1993). In

certain cases, as with the Agfa Horizon, a successfully scanned image must be transferred fromthe swap disk space to the disk space allocated to the users. For the Agfa Horizon thisprocedure needs 6 min for a 30 Mb image, so it is 3 times more than the time required for asuccessful scan. The digitisation of the image is just one part in the processing chain. Usuallyother processes follow, like ortho-image and DTM generation, mapping etc., i.e. proceduresthat require much more time than the scanning itself. As a conclusion, the scanning speedcould be much slower without any significant reduction in production throughput. Thereduction of the scanning speed would have several advantages: the scanning mechanism(mechanical, optical, etc.) could be slower which means simpler, cheaper and stablercomponents ; the integration time could be increased which means higher signal to noise ratio,and no need for powerful illumination which is expensive and generates a lot of heat,influencing the optomechanical and electronic parts, and requiring expensive and dust inducingcooling mechanisms ; vibrations in the scanning direction could be avoided or reduced ; thesmear in the moving direction depends on the product (scanning speed * integration time), so itcould be decreased if the integration time is increased less than the scanning speed isdecreased; noise like lag which is typical of high speed imagers could be decreased ; thebandwidth and the price of the electronics could be decreased while more operations could beapplied in “real-time” using hardware processing capabilities; large image buffers in thescanner that are required to store the data before transferring it to the host would not benecessary since the low data rate could be accommodated by the host/scanner interface or asmall image buffer.

1.2.7 The question of required geometric resolution

The required geometric resolution clearly depends on the application and the userrequirements. For DTM generation, there are many published good results that were achievedwith pixel sizes of 10 - 30 µm. Krzystek, 1992 reports accuracy comparisons for DTMgeneration between images scanned with 15 and 30 µm at a Zeiss/Integraph PS 1. Theaccuracy difference between the two versions was small and the 30 µm version had anaccuracy of ca. 0.1 ‰ of the flying height. For measurement of fiducials which can bemeasured with least squares matching with an accuracy of less than 0.1 pixel, a resolution of1200 dpi completely suffices, 600 dpi is also adequate. Problems may occur with signalisedcontrol points. With their standard size (optimised for the measuring mark of an analyticalplotter) a 1200 dpi resolution is almost the minimum for their accurate measurement. Forortho-image generation many people claim that the resolution can be fairly low, e.g. 20-40 µmor even lower. The argument they use is that for creating a sufficient quality hardcopy withaccuracy similar to those of printed maps or even lower this resolution suffices (Leberl,1992b). However, the digital ortho-images can be used, and there lies their value, inapplications that require higher resolution and accuracy, as in updating of topographic maps.For high geometric accuracy, a 600 - 1200 dpi resolution may be sufficient but for visualinterpretation an even higher resolution might be desirable especially in urban and forestedareas. Ideally the full resolution of the film should be reproduced, which given that highresolution films and aerial cameras with forward motion compensation can result in 60 Lp/mm(Meier, 1984) and assuming that to resolve a linepair ca. 3 pixels are required due to phasedifferences, this would require a pixel size of 5.5 µm and an image size of 1714 MB. However,

the visual interpretation can also be improved by other means than resolution, as digital imageenhancement, use of colour, oversampling, and stereo viewing. In addition, the overall imagequality does not only depend on pixel size. The quality depends on the characteristics of thewhole imaging system including illumination, optics, filters, sensor, A/D converter, hardwareand software processing etc. and to overestimate the importance of pixel size would be wrong.Although there are rapid developments in computer technology, huge data sets resulting from avery high scanning resolution (with 8 µm pixel size an aerial image would require 827 Mb) canstill not be handled conveniently or not at all. For this reason, owners of high resolutionscanners often do not use the highest resolution but a lower one that produces less data(Colomer, 1993, Kiefer, 1993). From a practical point of view, today the limit for a practicalhandling and interactive work seems to be around 15 - 20 µm. In the opinion of the author aresolution of 1200 dpi is sufficient for almost all measurement purposes. The main limitation isthe interpretation of fine details, and the signalisation of control points especially for highaltitude flights, but with the help of GPS this problem can be circumvented, or the orientationcan be performed at an analytical plotter. Le Poole, 1992 claims that the MTF of films is ratherthe same for different emulsions (grain sizes) with a value of around 50 % or less at 40 Lp/mm.Thus, he concludes that for a faithful digitisation a 10 µm pixel size is needed. Leberl, 1992aalso presents different arguments regarding the optimal pixel size and in Leberl, 1992b aslightly different opinion. Baehr, 1992 discusses the appropriate pixel size for ortho-imagesand proposes the use of variable pixel size within the same image and depending on theapplication. Diehl, 1990, discusses the effect of graininess on the radiometric noise in digitisedimages. He states that for 7.5 µm pixel sizes the radiometric noise due to graininess canamount to more than 20 % of the signal, while for larger pixel sizes it is much less. Hempeniusand Xuan, 1986 report on optimal parameters for digitising aerial images. Trinder, 1987mentions that a pixel size of 25 µm is adequate for pointing and interpretation, but systematicmeasuring errors of 0.2 pixels (however with manual pointing) require a pixel size of 10 µm inorder to preserve the precision of the image geometry. Trinder, 1989 uses synthetic images andreports tests on the influence of image quality, quantisation level, target size in pixels, and SNRon the precision of target location. Systematic errors can be significant if there is substantialasymmetry in the target intensity profile.

1.2.8 Literature overview

It is surprising how little has been reported up to now about the performance of scanners.As an example, although the Zeiss/Intergraph PS 1 was introduced in 1989 and Helava’sHAI 100 (DSW 100) even earlier, there is no published independent report on theperformance of these scanners as far as the author knows (with the exception of a very recentand brief report in Colomer, 1993). This is even more surprising since the cost of suchscanners is enormous, while for other much cheaper input devices, like CCD cameras, thereare hundreds if not thousands of published papers. In the following an overview ofpublications on scanners and related subjects will be given.

A survey of scanners (but with incomplete data and omitting many scanners) is publishedin Geodetic Information magazine, January 1993. Aslund et al., 1977 describe the IRIScomparator/scanner, and Hanf and Deter, 1983 the FEAG scanner/plotter, Brown, 1987 reports

on the Autoset scanner/comparator for industrial photogrammetry, Faust, 1989 the Zeiss/Intergraph PS 1 scanner, Leberl, 1990, 1992a, the VX-series of scanners. Gerhard, 1991, andKiefer, 1993, described the Signum HIRES scanner used in ortho-image production, Rollei,1993 the high precision réseau scanner RS1-C.

Accuracy analysis of scanners are published by Johannsen, 1976, and Lichtner, 1981.Boochs, 1984 reports on tests and accuracy analysis of drum scanners mentioning that theyexhibit significant geometric errors. A comparison of flatbed DTP scanners is given byMatazzoni, 1991, Diehl and Edwards, 1992, and Seiter, 1993. Bosma et al., 1989 give anevaluation of low-cost scanners, Steiger, 1991 make a quality comparison between colourDeskTop Reproduction and Electronic Image Processing scanners. Sarjakoski, 1992 reports onthe Sharp JX 600 DTP scanner and the development of a calibration procedure with anaccuracy of 0.2 pixel (8 µm) - he also reports in other tests an accuracy of 0.1 pixel. His resultsare too optimistic because all blunders larger than 30 µm had been removed (actually oftenerrors of +/- 60 µm occurred). Klaver and Walker, 1992 report that the same scanner withoutcalibration has sometimes local errors up to 170 µm. Chen and Schenk, 1992 present anextension of the DLT for calibration of linear and area CCDs, and scanners. Huurneman, 1992presents a low-cost transparency scanner made from off-the-self components, its calibrationand its accuracy.

Guelch, 1986, Baltsavias, 1988, Wilkins 1990, Fuchs and Ruwiedel, 1992, deal with CCDsmounted at analytical plotters, their use as scanners, and their calibration.

Thompson and Quellette, 1972 discuss the accuracy requirements of scanners. Makarovicand Tempfli, 1979 report on image digitisation for automatic photogrammetric processing.Gruen and Slater, 1983 present a strategy for testing the radiometric and geometricperformance of high resolution scanners. Ehlers, 1991 reports on digitising andphotogrammetric requirements. Diehl, 1992, discusses the optimal digitisation steps for usualfilms. Boberg, 1992 presents results from a subjective evaluation of image quality, and reportsthat image sharpness had the largest influence on quality assessment, while mean density,contrast and granularity had a smaller influence. Schroeder, 1992 discusses the spatialresolution of a linear CCD in terms of the optical transfer theory. Baehr, 1988 presentsdifferent methods for measuring the geometric resolution of digital cameras in general and inrelation to specific applications. There are hundreds of papers on geometric and radiometricanalysis of CCDs and many of them are useful, as most scanners employ linear or area CCDs.Because of space limitations we will restrict ourselves to Beyer, 1992 where many radiometricand geometric tests are presented, as well as a large reference list.

2. DESCRIPTION OF AGFA HORIZON

The basic characteristics of Agfa Horizon are:

• Scanning system: 3 optically butted linear CCDs Toshiba TCD 141 (3 x 5,000 pixels)

• Scan mode: colour (3 x 8 or 10 bit/pixel), grey level (8 or 10 bit/pixel), line-art (1 bit/pixel), halftone (Unix software supports only 8 bit/pixel for colour and grey level)

• Scan resolution: 20 (50 for the Unix-based system) - 1200 dpi, interpolated 2400 dpifor line-art and grey level (latter case not implemented in the Unix-based system)

• A/D conversion: 12 bit

• Internal image memory: 8 Mb, optionally 32 Mb

• Originals: reflective (any thickness, 297 x 420 mm), transparent (217 x 340 mm, withfirmware change 242 x 360 mm)

• Illumination: 2 halogen lamps, 400 W

• Lens (Agfa made): 3/1 object magnification, 107 mm focal length

• Density: range 3D, maximum density detected 3.3D ; auto density control, set blackand white point, editable tone curves

• Interface: SCSI-2 ; image passing modes to the host: FIFO or start/stop/transfer modewith multiple partial scans

• Computer support (incl. software): Mac (Photoshop, Letraset ColorStudio, QuarkX-Press, with a Desk Accessory scanning can be started from any Macintosh pro-gramme), PC (PC View Color), Sun (PRESS View Color, Pixel!FM)

• Scanning time: 2 min (real time) for 30 Mb for scanning, transfer, storage on swapdisk space and display (on Sun Sparc 2); scanning speed 5 - 100 mm/s (max speedthat can be realised for grey level images is ca. 20 mm/s); integration time 1 ms

• Driving system: 400 step stepping motor

• Colour scanning: 3 passes, dichroic filters with > 75% transmission

• Other characteristics: calibration of the illumination before each scan ; user-definedtone curves and unsharp masking in real time ; lifespan 7 years ; IR, IR/UV filters ;preview and scan area selection ; TIFF, Sunraster, EPS, Icon file formats ; descreen-ing ; two optional slide holders (24 x 35 mm, 6 x 6 cm etc.)

• Price: 41,500 SFr. + 3,700 SFr. for software + 6,000 SFr. for 32 Mb memory option.

The software related characteristics are applicable for the PRESS View Color Sun-basedsoftware, driver and firmware. There are some other functions that can be executed via existinghardware in real-time but they have not been implemented yet, e.g. horizontal and vertical flip,reverse video etc. After many years of development Agfa does not support any more PRESSView Color. As alternative software package for Unix-based systems, Pixel!FM of thecompany Mentalix can be used. It may even support workstations other than Sun.

Figure 3 shows the main components of the Horizon.

Figure 3. Horizon main components (front view): a. adjustable mirror, b. mirror carriage,c. lens and diaphragm, d. beam splitter with three CCDs, e. lighting unit.

Figure 4 shows the beam splitter. It is a firm hybrid component consisting of 3 CCDs anda beam splitting mirror. The beam splitting mirror provides an identical image in twoperpendicular planes. The CCDs are glued to the beam splitter with high accuracy ( 2

m).

Figure 4. Beam splitter.

Figure 5 shows the main components of the zoombox transport system (the main steelcable and the return wheel are not shown). The zoombox is one large optical blockweighting ca. 10 kg, which can be moved over the total A3 length. It contains a lighting unit,

±µ

mirrors, a colour filter wheel, a lens plus diaphragm, the beam splitter, one analogue videoboard, motors, sensors and one motor control board. The zoombox glides through the mainhousing on two skates, which are guided by the gliding bar and are driven by a stepper motorand a cable system. Positioning is controlled by the motor steps. The positioning accuracy isca. 10 m. The steel cable is tensioned by a spring and twisted around the motor axis. Thezoombox skates are two large blade springs that press the zoombox against the glass plate inorder to obtain a uniformly focused image across the scan area. The friction blocks of theskates glide at the bottom of the scanner housing over two guiding strips with a minimum oferosion. At the top, wheels attached to the zoombox ride against the glass plate.

Figure 5. Zoombox transport components: a. zoombox carriage, b. linear ball bearing, c.gliding bar, d. cable bearing wheel, e. main drive spring, f. horizontal returnwheel, g. main motor.

µ

Figure 6. Main circuit boards and video data stream.

Figure 6 shows the main circuit boards and the video data stream. The analogue videosignals from the 3 CCDs are multiplexed and amplified on the AMX board, which isattached to the zoombox. Six daughter boards (for even and odd sensor elements of the 3CCDs) monitor the video signal quality for dark signal drift and noise reduction. A videocoax cable transports the video signal at a rate of 15 MHz to the ANA board in the lowersection of the Horizon. The black and white corrections are applied using information fromthe shading RAMs. The correction values are stored in the RAMs by a calibration procedurewhich is executed before each scan. They are read during each scan line and are added to theanalogue video signal after 12-bit D/A conversion. The black correction is related to the darkcurrent and dark current differences among the sensor elements. The order of magnitude ofthis error is ca. 1%. It is visible as vertical stripes in the dark shades of an image and dependson the amount of exposure. The white correction relates to the different light sensitivities(gain) of the sensor elements. In addition, the illumination is never 100% uniformly spreadover the total scan line. Dust may also be a cause. The order of magnitude of this error is ca.10% and it is visible as vertical stripes in the low and medium density ranges of the image.

The pre-scaler multiplication controls the necessary amplification of the video signal, and isa function of lamp intensity, scanning speed etc. After a 10 bit A/D conversion, the ASICZORO demultiplexes the video signal and performs down-zooming of the image to a coarserresolution. Electronic zooming is an alternative to optical zooming and has the advantagethat the lens - CCD distance remains fixed. It has however other disadvantages as it will beshown in point 7 of section 3.1.) Unsharp masking (USM) for edge sharpening can beapplied in real time by using a 3 x 3 filter mask and adding the inverted second derivatives ofthe grey levels to the original signal. USM is locally adaptive and proportional to thecontrast over grain ratio. The contrast enhancement is applied by amplifying the highfrequencies of the signal more than the lower ones. Through bilinear interpolation a higherresolution can be achieved. The screener can convert the image to a 1-bit halftone image forprinting purposes. The digital video signal is clocked at 15Mpixels/s (without processing).

Figure 7. Left: original image. Middle: after edge enhancement (USM). Right: after moreedge enhancement.

The existing software (PRESS View Color) is generally positive. It is easy to use, has thebasic modules for input/output of images, scanning, image handling and display, processing,and image and colour editing. Scanning is performed easily and fast. The automatic densitycontrol works very well, and user-definable tone curves (including inversion andbinarisation) can be downloaded to the scanner and applied in real time. The only problem inscanning is that it may be interrupted either by an inexplicable error message (whichoccurred seldomly) or because the illumination is automatically turned-off to avoidoverheating. The processing routines are rather primitive and include sharpening,smoothing, noise removal and edge enhancement. Their effect is coarse, and the edgeenhancement (USM) creates a lot of artifacts at the edges (Figure 7). A positive aspect ofthese routines is that they can be applied to ROIs (closed contour with an arbitrary shape)with an undo possibility. Gamma corrections, inversion and binarisation can also be applied

a posteriori. Software for vectorisation of contours and lines using a Bezier or linearapproximation exists, but preliminary tests showed a poor performance, slow execution andoften execution failure. Basic, very useful features that are missing include histogramcalculations and measurement of pixel coordinates. Image handling and display areconvenient with the exception of the zoom factor which is limited when a maximum imagesize is reached, i.e. one can not zoom a lot with large images. The memory management isnot efficient. After scanning and display, the images are stored in the swap disk space. Sincethis is limited (in our case 320 Mb), one can not scan continuously images because even ifthe images are in between stored on the user disk space, their space in swap disk space is notfreed. Thus, in order to continue scanning one has to exit the program, the swap space iscleared (a very slow process with this software), and the program has to be started again. Alast problem is the fact that the image storage is very slow. The storage rate on a Sun Sparc 2(scanner host) over the network to the Sun server is 5 Mb/min, slower than our ownprograms by a factor of ca. 10.

3. TESTS OF AGFA HORIZON

Scanners, even DTP ones, are extremely complex and sensitive devices. Different sourceslike radio frequencies, magnetic fields that cause electrical noise, power supply etc. can have agreat influence on the scanner performance. Even if these possible error sources are ignored,there are still a lot of components to be checked and calibrated. In the case of Horizon differentcalibrations are performed: automatic calibrations in factory, service calibrations (start-up orperiodic), and calibrations before each scan. Each one of them consists of several calibrationprocedures, e.g. in service calibration twelve different components are tested and calibrated.Both hardware and software calibrations are executed. All software calibration results arestored and the corrections are performed before or during scanning, i.e. no softwarepostprocessing of the images is performed. Some calibration procedures are not supported fortransparent material. For calibration Agfa is using scan patterns, patterns that are permanentlywithin the scanner, and different calibration devices. The calibration patterns are in certaincases not stable or accurate enough, particularly for an accurate geometric calibration. To givean idea of the possible amount of errors it is enough to mention that Agfa lists 29 topics underservice trouble shooting, and 14 under image trouble shooting, most of them with manypossible causes and correction measures. This number may be frightening but it also showsthat Agfa paid attention to calibration, and at least the possible errors are known, even if theyare not always corrected. This is also proven by image quality specifications and tolerancesgiven by Agfa on 19 topics. In our experience most of these tolerances are met.

The tests were performed in 3 stages. Firstly, before buying the scanner by using thescanner of the Agfa office in Zurich. Unfortunately, due to limited swap disk space, largeimages and test patterns could not be scanned with 1200 dpi. Thus, some errors were notdetected at this stage. The second and most extensive test was after buying the scanner. Someof the occurring errors were, according to Agfa, due to this specific scanner that was bought, sothe scanner was replaced and the tests were repeated. The type of errors remained the same.Their magnitude, however, was different but their variations were not large. These variations

are due to scanner instabilities and fabrication tolerances. No specific tests were performed inorder to check the repeatability of the errors, but since the tests were extensive, were performedat least twice and extended over a period of several months some conclusions on errorrepeatability can be drawn. The tests concentrated on scanning transparent material, mainlygrey level, and to a much lesser extent colour. The tests will be divided in radiometric, andgeometric tests.

a) b) c)

Figure 8. Test plates from Heidenhain.

For testing we used aerial films of high dynamic range (both B/W and colour), some self-produced test patterns (see Figure 19), and the following commercially available test plates.A test plate of a Wild STK comparator with 23 x 23 grid lines with 1 cm interval and athickness of ca. 40 m was the main test pattern. The coordinates of the grid nodes weremeasured at a Wild AC3 analytical plotter. High quality test plates from Heidenhain (Dr.Johannes Heidenhain GmbH, Dr.-Johannes-Heidenhain-Str. 5, D-8225, Traunreut,Germany) were also used (see Figure 8). Plates 7 a) and 7 b) were used mainly to check thegeometric resolution, so their use is partly overlapping. Plate 7 c) is useful to check thevisual appearance of important features like lines and dots, in relation to the feature size.Some other test patterns to check the radiometric quality and colour quality were availablebut were used to a limited extent (see point 11. in section 3.1.). Finally, some of the testswere performed without any material, i.e. just the scanner glass plate was scanned. Othercompanies that sell different test patterns, even custom-tailored are Teledyne Gurley (514

µ

Fulton St., Troy, NY 12181, USA) and Max Levy Autograph Inc. (220 West Roberts Ave.,Philadelphia, PA 19144-4298, USA).

In scanner tests the highest possible true geometric resolution should be used, otherwisesome errors are not visible, or can not be quantified precisely. Additionally, for radiometrictests it is useful (i) to choose the density range of the scanner such that errors are amplified,and (ii) use postprocessing functions (gamma corrections, contrast enhancement) again inorder to amplify the errors and make them visible. After the errors are detected, theirquantification can be based on images scanned under “normal” operational conditions.

3.1. RADIOMETRIC TESTS

Radiometry is often underestimated in scanner testing, especially in the photogrammetriccommunity. However, good radiometric performance is of major concern even for expensivephotogrammetric scanners as Figure 1 and Figure 2 show. The here presented tests are notcomplete and include the following.

1. Photo Response Non-Uniformity (PRNU)

It was tested by scanning homogeneous areas (the scanner glass plate, or the test gridplate). In these uniform areas the standard deviation of the grey levels was computed andfound to be 1.5 - 2 grey levels. Care was taken to avoid areas including dust etc.

2. Vertical stripes

In uniform areas, black and particular bright vertical stripes can occur (see left part ofFigure 9). The grey level difference of these stripes from the neighbourhood is however verysmall, in the order of 2-3 grey levels. More serious problems can occur, if dust exists on thecalibration slit. This slit is used for a pre-scan calibration and automatic density control. Ifdust exists on the slit, then wrong correction values are computed for the underlying sensorelements, and very noticeable vertical stripes occur, having a width proportional to the widthof the dust particles (see right part of Figure 9). The slit can be cleaned from above, but dustcan be deposited in the lower part of the calibration slit (interior of the scanner), because the

scanner has openings for the cooling fans. The same error will clearly occur if dust exists onother parts like sensor elements, mirrors or lens.

Figure 9. Vertical stripes and horizontal banding (exaggerated for better visualisation).

3. Horizontal banding

This error is again visible in uniform areas (see Figure 9) but has a small magnitude.Agfa mentions as probable cause of this error, the lack of installation of a “power π” filter,but it is unknown to us what this means.

4. Echoes

Echo or ghost images are parts of an image that are repeated in another part of the image.This error is mainly due to the multiplexed read-out of the signal, i.e. the sequence of thepixels is pixel 1 of CCD1, pixel 1 of CCD2, pixel 1 of CCD3, pixel 2 of CCD1, pixel 2 ofCCD2 etc. This means that adjacent information in the video signal does not refer toadjacent elements in the original image. This may cause sharp transitions in the analoguesignal. When electronic circuits begin to fail, echo problems may occur. Another causemaybe the unwanted reflection of light in the optical path (secondary reflections). Echoesmay be visible and occur at sharp edges. In the case of multiplexing, the edge will berepeated in all three partial images (one from each CCD). The error will be particularly

visible when a bright object in one CCD falls on a dark area in the other two CCDs and viceversa. Examples of such errors are shown in Figure 10.

Figure 10. Echoes. Top: border of a scanned aerial film (full width covering all 3 linearCCDs). Middle left: the letter S is repeated to the left and right of its position(error due to secondary reflections). Middle right: LK 225 and 6298 comingfrom the 1st and 2nd linear CCD are repeated in the 3rd CCD (error due to mul-tiplexing). Bottom: the multiplexing error occurs even if a bright object fallsonto bright background (here the image is stretched with a gamma of 0.05 forvisualisation purposes ; the multiplexing error in the original image was ca. 1grey level).

5. Dynamic range

As a test, films with high dynamic range were scanned. The films included the snow-covered Alps, shadow areas, dark forested areas all in the same image simultaneously. Inaddition, the very dark border of the film was also scanned. Automatic density control wasused and the result was checked visually and for saturation. By changing the LUT ofindividual grey levels, e.g. 0 and 255, and displaying them in color saturated areas can beeasily detected. However, Horizon gave very good quality images without saturation, forimages with a density range of over 2D.

6. Radiometric differences along the borders of partial scans

This error is related to a geometric error and will be treated in the section on geometrictests.

7. Subsampling errors

The horizontal scanning always occurs at 1200 dpi. For down-zooming (subsampling)the signal is first low-pass filtered and then resampled (fast interpolation in hardware). Invertical direction, down-zooming is obtained by speed control of the zoombox. Scannersthat do not have an optical zoom facility (with the exception of scanners employingmicroscanning) use similar procedures for subsampling, whereby the area based CCDs uselow-pass filtering and resampling in both directions. This way of subsampling createsdifferent problems. Horizontal lines may totally disappear if they lie in the gap between twoconsecutive positions of the CCD lines. Since low-pass filtering and resampling occurs onlyin horizontal direction, the two directions are treated differently. To check these problemsthe test plate of Figure 8 c) was used. The plate has 17 lines/dots of width/diameter from 100to 3 µm. The distance between the lines and their length is 2 mm. Figure 11 shows the platewith the lines in horizontal direction (the displayed images has been rotated by 90 degrees)and a scanning resolution of 600 dpi. The top image of the Figure shows a grey level profilealong the middle of the lines. Since the lines are of decreasing width, the contrast of the linesshould monotonically decrease. This is not exactly the case, but approximately so. This canalso be observed in Figure 14 that shows the modulation M of the lines ( ).The bottom image of Figure 10 shows a profile along a line with a uniform grey leveldistribution along the line. So, when subsampling by factor two, no significant errors occur.All lines are visible, although with a continuously decreasing contrast as the line thicknessdecreases. The dots are visible up to 20 µm size. Figure 12 and Figure 13 show similarresults for a scanning resolution of 150 dpi and horizontal and vertical lines respectively. InFigure 12 some horizontal lines almost disappear, as it was expected, since they lie in thegap of 148 µm between two consecutive positions of the CCD lines. As Figure 14 shows themodulation is not monotically decreasing with decreasing line width. The 50 µm thick linehas a higher contrast than the 100 µm line, while the 10 µm line has a similar contrast as thesame line scanned with 600 dpi! Thin lines of even 3 µm width are visible because thecontrast of the lines of the glass plate is high and the effective pixel dimension in thescanning direction is 21.2 µm (1200 dpi). The grey level profile along a line is homogeneousas it can be seen in the right image of Figure 12. The dots are visible up to 30 µm size. Theresults of Figure 13 are unexpected and difficult to explain. The top image of the Figure aswell as Figure 14 show a very nonmonotonic modulation. The 20 µm thick line has a similarcontrast as the 100 µm line, and a much higher contrast than the same line scanned with 600dpi! Many lines are hardly visible (actually the horizontal lines of Figure 12 are bettervisible, contrary to the expectations). There is no explanation for this other than that the low-pass filtering and resampling have not been implemented properly. The bottom image ofFigure 13 also shows that the grey level profile along a line has many discontinuities. Thismay be due to the fact that neighbouring pixels along the line are actually 169 µm apart dueto the gap between two consecutive positions of the CCD lines. Only the dots of 100, 30, 25,and 20 µm size are visible.

MImax Imin–

Imax Imin+---------------------------=

As a conclusion subsampling by a factor more than 2 should be avoided. Actually, theideal solution is to always scan with full resolution and then use an image pyramidgeneration algorithm for a proper subsampling.

Figure 11. Top: grey level profile along horizontal lines (displayed image has been rotatedby 90 degrees) of different thickness scanned with 600 dpi. Bottom: grey levelprofile along one line.

Figure 12. Left: grey level profile along horizontal lines of different thickness scannedwith 150 dpi. Right: grey level profile along one line.

Figure 13. Top: grey level profile along vertical lines of different thickness scanned with150 dpi. Bottom: grey level profile along one line.

Figure 14. Modulation (contrast) for lines of different width and different scanning resolu-tions. Top left: horizontal lines (in CCD direction) with 600 dpi. Top right: hor-izontal lines with 150 dpi. Bottom: vertical lines (in scanning direction) with150 dpi.

8. Oversampling errors

Oversampling should theoretically be possible for B/W and grey level images. WithPRESS View Color however it can be performed only for B/W images. Even this operationdoes not function properly as Figure 15 shows. There are vertical streaks every two lines,and the lines with the error in the upper part of the thick black line of the right image ofFigure 15 are shifted in x-direction by 1 pixel in comparison to the lines with the error at thelower part.

Figure 15. Left: binary image scanned with 1200 dpi (twice enlarged). Right: binary imagescanned with 2400 dpi.

9. Smear

It is caused by the movement of the zoombox in vertical direction and it is proportionalto the scanning speed. Thus, sharp edges are defocused, i.e. their contrast is decreased andthey become wider (Figure 16). Agfa mentions a 20% loss of focus in the scanning direction,whereby in our results with the grid test plate, the vertical lines were 2 pixels wide, but thehorizontal lines 2.5 - 3 pixels. This smearing also decreases the resolution in the scanningdirection.

Figure 16. Smear of horizontal lines

10. Different noise patterns between the 3 CCDs

CCD line direction

Scanning direction

This effect has been observed, e.g. in the 4 linear CCDs of SPOT. For this purpose ahorizontal stripe of the grid test plate was scanned. A low-passed filtered version of thisimage was subtracted from the original to detect the noise, and the result was normalised.The noise pattern in the vertical swath of each CCD was visually inspected but no differenceamong the 3 CCDs was found.

11. Other image quality tests

For this purpose different test patterns can be used. The UGRA/FOGRA (UGRA/FOGGRA, 1988) reproduction test chart is a 20 x 27 cm photograph (cost 250 SFr.) whichcan be used for control of tonal rendering, colour rendering, gray balance, imagereproduction, and image quality, e.g. defects such as dominant colour casts, improper greybalance, graininess etc. Kodak sells photographic step tablets (transparencies) with up to 21density steps in 0.15 spacing and a 250 mm length, and colour separation guides and grayscales. A gray scale with 20 density steps with 0.1 spacing (0 - 1.9D) was scanned with theHorizon. The results are not conclusive because the grey scale was old and partly dirty andbecause for a good test of the radiometric quality much more than 20 density steps areneeded. In any case, the automatic density control determined almost exactly the minimumand maximum density and the density difference between neighbouring steps was generallycorrect.

Concluding, Horizon has a pretty good radiometric performance with the main errorsbeing the echoes due to multiplexing, the smear in the scanning direction (not a radiometricproblem in principle) and the dust problems which are however a problem common for mostscanners.

3.2. GEOMETRIC TESTS

The following geometric tests were performed.

1. Nonalignment of the 3 CCDs

For this purpose the horizontal lines of the grid test plate were used (actually one linesuffices) and scanned over the whole possible width. Knowing the approximate position ofthe transition from one CCD to the other the horizontal line was checked for discontinuities.If a discontinuity was visually detected, then the vertical position of the line on either side ofthe discontinuity was determined using lest squares template matching and the misalignmenterror was quantified. In the old scanner, the error between two of the CCDs was 0.3 pixels(Figure 17), in the new scanner no error was observed. This type of error is more or lessconstant over time. It can not be corrected so easily in real time using hardware because itrequires transformation and resampling in both CCD and scanning directions. The same testpattern can be used to detect more general errors caused by noncolinear CCDs. However,overlap errors can not be detected by this pattern. Only about 4,777 sensor elements fromeach CCD are used for the image composition. An error in the correct detection of the lastactive pixel of one CCD and the first one of the next CCD will lead to gaps or overlaps, i.e.global shifts of all elements of the next CCD, and scale differences. For calibration, patterns

similar to those mentioned in 4. below (i.e. inclined lines) or two known distances (e.g. 3crosses, one for each CCD) can be used.

Figure 17. Misalignment between two linear CCDs.

2. Vibrations

Vibrations in horizontal directions were detected using the grid test plate. The vibration(shift) was constant within each scanline. The vibrations caused the straight lines to beimaged as a sine curve (Figure 18). The vibrations do not occur continuously along the scandirection and they tend to be more frequent and pronounced towards the end of the scan. Inthe old scanner the maximum amplitude of the sine function was ca. 0.5 pixels, in the newscanner 0.1 - 0.2 pixels. This error is particularly disturbing because it is not stable, so itmust be corrected for each scanning. As test pattern a thin line in the vertical direction alongthe image border can be scanned (see Figure 19). The line must not be known, but it musthave high contrast and must be straight. Using image processing the edge points can bedetected, a straight line fit can be computed, and the residuals from this fit will give theposition and the amount of the errors. These errors can be corrected a posteriori by a lineartransformation and resampling in the horizontal direction only for the lines where suchvibrations occur.

Figure 18. Vibrations in horizontal (CCD) direction (displayed image has been rotated by90 degrees).

Agfa mentions also vibrations in the vertical direction. We could not detect suchvibrations. Possible reasons for that are: (i) the smear in the vertical direction which couldmask the vibrations, and especially (ii) the fact that the spacing between the horizontal linesof the grid test plate was too large (1 cm), so vibrations which could have occurred betweenthe lines can not be detected with this test pattern. Vertical vibrations cause nonuniformmovement in the scanning direction, effect which can also be caused by a poor mechanicalpositioning mechanism.

3. Lens distortion and scale differences in horizontal direction

To check this, the grid test plate was scanned. The distances between the grid nodes atthe left and right border, and at the center of the grid plate were computed after estimatingthe grid node position by least squares template matching. At the borders a distance of 1 cmwas 469.5 - 470 pixels, while at the center 475 pixels. This corresponds to pixel sizes of21.30 - 21.28 µm, and 21.05 µm respectively, while with a 1200 dpi scanning a pixel size of21.17 µm should be expected. These errors although large, are relatively stable over time,and in theory they could be corrected in real-time by using the Horizon hardware. After acalibration, correction values (horizontal shifts) could be saved in a LUT and a lineartransformation and resampling could be performed.

Figure 19. Self-made test patterns used for correction of vibration and partial image scan-ning errors (displayed image has been rotated by 90 degrees).

4. Partial image scanning

This is a peculiarity of Horizon. When the image to be scanned is larger than the imagebuffer of the scanner (32 Mb in our case), then a partial image with size equal to the framebuffer is first scanned, the zoombox moves to the start position, the image data aretransferred to the host, the zoombox is positioned after the end of the previous partial scan,the next partial image is scanned, and the procedure is repeated until the whole image isscanned. These partial images have an overlap. To scan an aerial image with 1200 dpi 4-5partial images are needed. The overlap error was quantified by using a self-made patternconsisting of high contrast lines, approximately straight, with an inclination of 45 degreeswith respect to the horizontal direction (see Figure 19). At the position of the overlap errorthe lines were broken (see Figure 20), and by using again least squares template matching(other image processing procedures could also have been used) the error could be quantified.The error was typically 2 pixels between 1st and 2nd partial image, 3 pixels between 2ndand 3rd partial image, and 3.5 pixels between 3rd and 4th partial image.

Figure 20. Test pattern and overlap error. From left to right between 1st and 2nd, 2nd and3rd, 3rd and 4th partial images.

Although there were small variations the error magnitude was fairly constant in both oldand new scanner, and was always increasing towards the end of the scan. Apart from thegeometric error there were also small, but in many cases visible, radiometric differences atthe border between two partial images, due to illumination instabilities. Our interpretationwas that the scanner had not a precise enough positioning system, and such errors are knownto increase with increasing distance from the zero point of the transport mechanism. Aftermonths of discussions with Agfa, and possible explanations from their side which were allwrong, we were informed that this problem occurs only with the Unix-based systems and isdue to the driver. To test this, a grid test plate was scanned at a Mac-based Horizon with 8Mb image buffer at the Federal Office of Topography in Bern. With 8 Mb image buffer, thereare a lot of partial images, and the overlap error between neighbouring images may not bevisible. However, if this error would have occurred, then distances between an increasingnumber of partial images would have been shorter and shorter. A comparison of the knowndistances between grid nodes in the vertical direction, with the distances measured in theimage, indicated that no overlap error occurred. Whether this statement can be generalised,is unknown.

5. Colour misregistration

To test the colour registration between the R,G,B channels high contrast B/W edges invertical and horizontal direction are useful. Thus, the test plate of Figure 8 a) was used. Inthe old scanner the misregistration was clearly larger in vertical direction, due to mechanicalpositioning errors. Although Agfa gives a tolerance of 1 pixel, the error in vertical directionwas ca. 2 pixels. A quantification of the error is possible with visual, manual measurementsand even better by separating the three channels in 3 files and then measuring correspondingfeatures, e.g. corners, in all three channels. In the new scanner, the situation was opposite.The error in the vertical direction was in the 1 pixel range, but in the horizontal direction itwas ca. 2 pixels. Possible error sources include the lens (unknown whether it isapochromatical) or differences in the colour filters, each of which acts as a lens.

6. Geometric resolution.

The test plate of Figure 8 a) was visually inspected and the smallest group of 3 line pairsthat could be discerned was detected. This corresponded to 17 Lp/mm (i.e. 29.4 µm linewidth) for the vertical lines and 20 Lp/mm (25 µm line width) for the horizontal ones, whileafter unsharp masking it was increased by factor 1.12 or 1.26. Our expectation was that dueto smear the resolution would be better for the vertical lines. The plate of Figure 8 b) wasalso used to determine visually the resolution in the vertical direction, and in this case it wasfound to be 16.67 Lp/mm (i.e. 30 µm line width). This result, that to resolve a certain signalfrequency ca. 50% higher sampling frequency is needed, is in accordance with theexpectations and is due to phase differences that may occur between signal and samplingfrequency.

7. Deformations

This effect was observed only once, when scanning a poor quality aerial film. At certainregions of the image large deformations were observed. After a slight movement of the filmand rescanning, the deformations still existed, but they were different (see Figure 21). Thereis no clear explanation of this error. However, since at the regions where the deformationsoccurred, the lines of the grid test plate that was covering the film were not deformed, itseems that the error was due to film deformation.

Figure 21. Top: deformations (probably due to film deformation). Bottom: same as top butafter slightly moving the film and rescanning. Deformations still exist but aredifferent. Note that the lines of the grid test plate are not deformed.

8. Global geometric errors

For this purpose, all grid nodes of the grid test plate scanned with 600 dpi wasdetermined by least squares template matching and compared to the known values. In total492 visible crosses were used. An affine transformation between the two data sets, using allcrosses as control points gave an a posteriori standard deviation of unit weight of 52 µm. Aquadratic transformation gave a respective value of 47 µm, and the x2, xy, and y2 terms inhorizontal direction were not significant. This indicates that an increase of the degree of thepolynomial does not necessarily lead to large error reductions, and that in scanning directionhigher order terms are required. The same procedure was repeated by leaving out the threeborder columns of the crosses (both left and right). In this case the a posteriori standard

deviation of unit weight was reduced significantly to 31.5 µm and 24.4 µm for the affine andquadratic transformation respectively. This indicates significant errors due to lens distortion.

To simulate a realistic situation of aerial image scanning where only the fiducial marksare used for the interior orientation, we computed an affine and a bilinear transformation,using as control points the 4 corner crosses. The results of the two transformations were verysimilar, whereby the xy term of the bilinear transformation in the horizontal direction wasnot significant. Two cases were compared. One by using the 4 corner crosses, most of whichwere poorly imaged (case A), and a second one by using other better defined crosses in theneighbourhood of the corners (case B). In case A the RMS error was 100 µm in x and 84 µmin y, and in case B 87 µm in x and 78 µm in y. This error magnitude is what should beexpected from Horizon without any geometric calibration. With 1200 dpi scanning the errorwould decrease only insignificantly because the error sources and magnitude remain thesame. With a higher scanning resolution, only the matching accuracy could be possiblyimproved but this gain would be very small compared to the whole error budget. In bothcases the residuals of the remaining 488 check points showed very strong systematicpatterns, particularly the x-residuals. Figure 22 shows the pattern of the residuals for the firsttwo grid lines. For the other lines similar patterns were observed alternating from line toline. The magnitude of the x-residuals remained nearly constant from line to line, while forthe y-residuals it was increasing towards the center of the image.

Figure 22. Systematic pattern of residuals of check points along horizontal grid lines.

No calibration software has been written yet for the Horizon. It is expected that aftercalibration the RMS errors will be reduced to under 10 µm. The errors in the CCD directionare relatively stable over time with the exception of the vibrations. The errors that arefrequently variable and thus difficult and time consuming to correct are in the scanningdirection. The overlap error is fairly simply to detect. The most difficult errors are due tomechanical positioning errors and vibrations that can cause local systematic errors likeoverlaps, gaps, different pixel sizes and scale differences. These errors are constant within

Horizontal (CCD) direction

Ver

tical

(sc

anni

ng)

dire

ctio

n

1st line

2nd line

x-residuals y-residuals

each scan line, and thus for their detection test patterns on either or both sides of the imagecould be scanned. These test patterns should include a vertical line for the detection ofvibrations and crosses in vertical direction with a small spacing. Only the second patternmust be geometrically precisely known.

4. SCANNER SPECIFICATIONS FOR PHOTOGRAMMETRIC TASKS

An optimal photogrammetric scanner should have the following characteristics:

• Flatbed stage (high geometric accuracy, scanning of rigid documents).

• Optically butted linear CCDs.

many elements per CCD, butted precisely, 3-chip CCD wished, low noise (dark cur-rent, fixed pattern, reduced smearing and blooming), no defect pixels, uniform re-sponse, high maximum charge storage capacity, read-out without multiplexing of thebutted CCDs (i.e. single readout register per CCD array), individual exposure controlfor each colour channel. An interesting technology is also Time Delay and Integra-tion (TDI) scanning which permits a higher scanning speed or a higher SNR (such atechnique is employed e.g. in the Polaroid CS 500 scanner).

• Calibration (radiometric and possibly geometric before each scan), corrections im-plemented in hardware as much as possible, calibration software and test patternsprovided, geometric errors constant over a long time period.

• Variable geometric resolution (with optical zoom or electronic zoom with correctsubsampling), highest resolution of at least 1200 dpi.

• 12-bit quantisation with freely definable reduction to 8-bit, density range greater than2.5D.

• Powerful, uniform, and stable, white illumination, preferably cold light. Avoidance offlare light particularly when scanning transparencies.

• Clean and stable power supply.

• Scan speed not very relevant (10 - 30 min for a 1200 dpi B/W scan) ; user definable.

• Preferably movable stage with all electronics, optics and illumination stationary.

• Flat, anti-Newton glass plates, help to position approximately the document on theglass plate, flat and stable cover.

• Simple optics (mainly filters and lens between illumination and sensor with minimi-sation of mirrors).

• Software (free setting of scanning parameters, editable tone curves, automatic densitycontrol, basic measurement, editing and image processing routines, support of vari-ous image formats incl. TIFF).

• Support by various computers, no need of large image buffer, no writing on swap butdirectly on user disk space, standard interface (SCSI).

• Accurate mechanical positioning in scanning direction (RMS = 0.25 - 0.33 pixel ofhighest geometric resolution, i.e. smallest pixel size).

• Chromatically corrected lens with low lens distortion (latter not so important, can becorrected by calibration, using hardware eventually in real time).

• No mechanical oscillations.

• Packaging such to avoid dust.

• Scanning of reflective and transparent, binary, B/W and colour documents.

• Scanning format (for aerial images 26 x 28 cm to allow scanning of calibration pat-terns at the film borders, to account for 30 x 30 and 23 x 46 cm satellite images theformat should be 35 x 50 cm). The glass stage and the illuminated area larger by atleast 1 cm on each side.

• Price below 100,000 SFr.

5. CONCLUSIONS

DTP scanners can be used for photogrammetric tasks, if they are calibrated, especially ingeometry. Some of their components like sensors, electronics, computer platforms, software,and characteristics like radiometric performance and speed are partly equal or better thanthose of very expensive photogrammetric scanners. The main problems of DTP scanners arepositional accuracy, uniformity and repeatability, mechanical instabilities (vibrations), andlens distortion or imperfections of other optical parts (mirrors, filters). Positional accuracy isnecessary to avoid overlaps or gaps when scanning in multiple swaths or multiple CCDframes, and colour misregistration errors when scanning in 3 passes. A uniform movementin scanning direction is required to avoid local errors that result in gaps, overlaps and thusalso scale differences. Nonuniformities can be caused not only by the positional mechanismbut also vibrations in the scanning direction. Even if these requirements are fulfilled,systemetic gaps or overlaps over the whole scan length can occur, if there is no exactsynchronisation between scanning movement and signal integration/read-out. Thepreviously mentioned problems can be severely reduced by employing better scanningmechanisms (usually mechanical) and lenses. However, even in this case a calibration willstill be necessary, as it has been shown by the errors occurring with expensivephotogrammetric scanners. The importance of high scanning speed is overestimated. Highscanning speed leads to several negative or expensive consequences.

Calibration includes modelling and correction of errors that can generally be divided intotwo groups:

1. Slow varying errors

These errors mainly occur in CCD direction and are related to lens distortion andscale differences. Other CCD sensor related errors like CCD misalignment and blem-ishes are also relatively stable. Calibration of this type of errors is easier as it can be

performed once every a long period of time. Some of the necessary corrections canbe applied in real-time during scanning by using off-the-self hardware that read pre-saved correction values and perform 1-D geometric transformations and grey levelinterpolation. This is particularly the case with linear CCD based scanners. DTPscanners like the Horizon already have such hardware, but it is not used for geometriccalibration.

2. Frequently varying errors

Such errors, like scale differences, mainly occur in scanning direction. Errors in CCDdirection, like vibrations, also exist. Finally, radiometric errors, e.g. due to illumina-tion instabilities, are in most cases frequently varying. This type of errors require adifficult and time consuming calibration, which must generally be performed for eachscan. Thus, test patterns must be scanned together with the images, and the correctionmust be applied after scanning. The size of the test patterns, e.g. thickness of lines,must be chosen appropriately so that the patterns can be measured precisely for allgeometric resolutions that are typically chosen by the user.

Such calibration procedures and test patterns should ideally be supplied by the scannervendors but this is unfortunately a rare case. However, this task can also be performed by auser, if the necessary expertise and means are available. Calibration is also a way of reducingthe scanner costs by avoiding expensive mechanical and optical parts, of course at a cost of alower scanning throughput.

The geometric accuracy of Agfa Horizon without applying any geometric calibration isca. 80 µm. This RMS error was estimated using 500 check points, four control points at thecorners and an affine transformation. The geometric accuracy potential after calibration canbe ca. 10 µm. It may be even lower, if the frequently varying errors are well modelled andcorrected for. The errors in CCD direction considerably increase towards the borders of theglass plate, and in scanning direction they increase slightly towards the end of the scan.Surprisingly, the errors in scanning direction, which are more frequently varying and moredifficult to correct are smaller than those in CCD direction. The type and the ordermagnitude of the oocuring errors is quite stable.

No errors occur only in the interior of the image, which makes a calibration possible byusing test patterns at the borders of the image. Actually, in most cases only one border ineach direction (horizontal and vertical) need to be used. In some cases the test patterns canbe self-made without requiring an exact knowledge of their geometric characteristics. Evenif the latter is a necessity, the patterns can be self-made and measured, e.g. at an analyticalplotter. If the base material is stable, e.g. film with thick Estar base, then this measurementprocedure must not be repeated very often.

The emergence of several standards in DTP make a brand selection for the user mucheasier because more products are compatible with each other. Examples include file formats(like TIFF or Kodak Photo CD), compression, colour management (such as Apple’sColorSync, interfaces (SCSI), and the relatively new TWAIN application program interface(API) which has been adopted by most scanning hardware and software vendors. The

TWAIN API aims to provide a seamless link between compatible products, meaning anyTWAIN-compatible software will drive any TWAIN-compatible scanner. In DTP thecomputers that are mostly used are Macs and to a lesser extent PCs. For this reason whenbuying a DTP scanner it is worth considering connecting it to such a computer because theyare supported more by the scanner vendors, and there are a lot of good and cheap programsrelated to the scanners.

DTP scanner vendors usually do not know, or if they know they do not care, about therequirements in the photogrammetric community. It is our task, and especially the task ofinternational and national societies, to contact them and conceive them about thephotogrammetric/cartographic/GIS market potential, so that they derive appropriatecalibration software and also possibly modify the scanners in order to better meet ourrequirements and ease calibration. The author tried for several months to convince Agfaabout the above necessity, unfortunately without success.

With the developments in the DTP scanner market and in low-cost photogrammetricscanners, it should be expected that several good quality scanners will soon be available at aprice under 100,000 SFr. Today there are various scanners in this price range, that can orcould be used for photogrammetric tasks. In the category of photogrammetric scanners theVX 3000 from Vexcel Imaging Corp., the RM-1 Rastermaster of Wehrli Inc., whileInternational Systemap Corporation has announced the Digital image Scanner (DiSC), ascanner with very good specifications but still unknown performance. The Rollei RS1-Ccould theoretically also be employed as a scanner but this was very rarely the case. In theDTP category with a minimum resolution of 1200 dpi and sufficiently large format forscanning aerial images, apart from Agfa Horizon, there is the Howtek Scanmaster D4000,and the newly announced Smart 340L from Scitex. It is a 1200 dpi, A3 format, CCD-basedflatbed scanner for the Macintosh which can scan reflective or transparent, B/W or colourdocuments. The Ricoh FS2 is a one-pass colour-CCD, 10-bit quantisation, 1200 dpi scannerof A4 format (according to another publication reflective documents of up to 23 x 37 cm canbe scanned) with transparency option, costing 6,300 DM. In the opinion of the author theexpensive photogrammetric scanners have no future. For photogrammetric tasks low-costphotogrammetric or DTP scanners will be used, whereby in the latter case the user should inmost cases develop calibration software himself.

ACKNOWLEDGEMENTS

The author has appreciated the friendliness, professional attitude, readiness to help andcooperation of several members of Agfa Zurich, especially Mr. H. Kiefer and Mr. Debonna.Thanks go to L. Hurnie, Institute of Cartography, ETH Zurich, who prepared the test patternof Figure 19. The cooperation of J.-P. Perret, Federal Office of Topography, Bern on testswith a Mac-based Horizon is also acknowledged.

REFERENCES

Aslund, N., Carlsson K., von Gresdorff N., Petersson L., 1977. Digitisation of Images byMeans of Fast Raster Scanning using the Vibrating Prism Facility of the Measuring Ma-chine IRIS. Proc. of Int. Symp. on Image Processing, Graz, 3-5.10.77, pp. 11-14.

Baehr, H.-P., 1992. Appropriate Pixel Size for Orthophotography. Proc. of 17th ISPRS Con-gress, Vol. 29/B1, pp. 64 - 72.

Baltsavias, E.P., 1988. Hierarchical Multiphoto Matching and DTM Generation. Proc. of 16thISPRS Congress, Vol. 27/B11, pp. III:476-492.

Beyer, H.A., 1992. Geometric and Radiometric Analysis of a CCD-Camera Based Photogram-metric Close-Range System. Ph. D. dissertation, Institute of Geodesy and Photogrammetry,ETH Zurich, Mitteilungen Nr. 51, p. 186.

Boberg, A.E., 1992. Subjective vs Objective Image Quality. Proc. of 17th ISPRS Congress,Vol. 29/B1, pp. 73 - 78.

Boochs, F., 1984. Die geometrische Leistungsfaehigkeit von Trommelscannern - Anspruchund Wirklichkeit. Proc. of 15th ISPRS Congress, Vol. 25/A2, pp. 68 - 76.

Bosma, M., Drummond, J., Raidt B., 1989. A Preliminary Report on Low-Cost Scanners. ITCJournal, No. 2, pp. 115 - 120.

Brown, D., 1987. Autoset, Automated Monocomparator Optimised for Industrial Photogram-metry. Proc. of Int. Conf. and Workshop on Analytical Instrumentation, published by AS-PRS, pp. 292 - 307.

Chen, Y., Schenk A.F., 1992. A Rigorous Calibration Method for Digital Cameras. Proc. of17th ISPRS Congress, Vol. 29/B1, pp. 199 - 205.

Colomer, J.-L., 1993. First Experiences Using Digital Photogrammetric Stereo Workstations atthe ICC. Photogrammetric Week’ 93, Fritsch/Hobbie (Eds.), Wichmann Verlag, Karlsruhe,pp. 185 - 195.

Diehl, H., 1990. Radiometric Noise in Digitised Photographs. Proc. of SPIE, Vol. 1395, pp.974 - 983.

Diehl, H., 1992. Optimal Digitisation Steps for Usual Film Materialss. Proc. of 17th ISPRSCongress, Vol. 29/B1, pp. 1 - 6.

Diehl, S., Edwards, D.L., 1992. Scanning the Spectrum. Byte, June, pp. 230 - 242.Ehlers, M., 1991. Digitisation, Digital Editing, and Storage of Photogrammetric Images. Proc.

of 43rd Photogrammetric Week, Stuttgart Univ., Inst. of Photogrammetry, Heft No. 15, pp.187 - 193.

Faust, H.-W., 1989. Digitisation of Photogrammetric Images. Proc. of 42nd PhotogrammetricWeek, Stuttgart Univ., Inst. of Photogrammetry, Heft No. 13, pp. 69 - 78.

Fuchs C., Ruwiedel S., 1992. Digitisation and Rectification of Transparencies with the Analyt-ical Plotter P3. Proc. of 17th ISPRS Congress, Vol. 29/B2, pp. 18 - 24.

Gerhard, A., 1991. Digital Orthoprojection: Scanning, Handling and Processing of Aerial Im-ages. Proc. of Conf. Digital Photogrammetric Systems, Ebner/Fritsch/Heipke (Eds.), Wich-mann Verlag, Karlsruhe, Germany, pp. 177 - 187.

Gruen, A., Slater, P.N., 1983. A Test Strategy for High Resolution Image Scanners. Report No.350, Dept. of Geodetic Science and Surveying, Ohio State University.

Guelch, E., 1986. Instrumental Realisation and Calibration of Digital Correlation with thePlanicomp. Proc. of 40 Photogrammetric Week, Stuttgart Univ., Inst. of Photogrammetry,Heft No. 11, pp. 91 - 108.

Hanf, R., Deter, C., 1983. Automatische digitale Bildverarbeitung, Analyse und Auswertungmit dem Film-Ein/Ausgabe-Geraet FEAG. Jenaer Rundschau, No. 4, pp. 176 - 179.

Hempenius, S.A., Xuan, J.-B., 1986. On the Equivalent Pixel-Size and Meaningful Bit-Number of Airphotographs, Optimal Scanning Aperture and Scanning Increment for Digi-tising Airphotographs. Proc. of ISPRS Com. I Symp., ESA-SP-252, pp. 451 - 464

Holch, K.M., 1993. Scanning in a Wonderful World. Computer Graphics World, October, pp.59 - 68.

Huurneman, G., 1992. A Low Cost Scanner for Small Format Transparent Material. Proc. of17th ISPRS Congress, Vol. 29/B2, pp. 25 - 30.

Johannsen, Th., 1976. Der Scanner - ein Geraet zum schnellen Digitalisieren inhaltsreicherVorlagen, Nachrichten aus dem Karten- und Vermessungswesen, Heft 70, pp. 13 - 20.

Kiefer, L., 1993. Digital Photogrammetry in the Land Consolidation Authority in Baden-Wuerttemberg. Photogrammetric Week’ 93, Fritsch/Hobbie (Eds.), Wichmann Verlag, Karl-sruhe, pp. 169 - 172.

Klaver, J., Walker A.S., 1992. Entry Level Digital Photogrammetry: Latest Developments ofthe DVP. Proc. of 17th ISPRC Congress, Vol. 29/B2, pp. 31 - 37.

Leberl, F., 1990. The VX-Series of Interactive Film Scanners: Film-Based Softcopy Photo-grammetry. Proc. of SPIE, Vol. 1395, pp. 299 - 307.

Leberl, F.W., 1992a. Photogrammetric Scanning with a Square Array CCD Camera. Proc. of17th ISPRS Congress, Vol. 29/B2, pp. 358 - 363.

Leberl, F.W., 1992b. Design Alternatives for Digital Photogrammetric Systems. Proc. of 17thISPRS Congress, Vol. 29/B2, pp. 384 - 389.

Le Poole, R.S, 1992. Potential and Limitations of Digitisation of Photographs. Geodetical InfoMagazine, May, pp. 58 - 60.

Lichtner, W., 1981. Anwendungsmoeglichkeiten der Rasterdatenverarbeitung in der Kartogra-phie. Wissenschaftliche Arbeiten der Fachrichtung Vermessungswesen der UniversitaetHannover, Heft 105, 72 p.

Makarovic, B., Tempfli, K., 1979. Digitising Images for Automatic Processing in Photogram-metry. ITC Journal, No. 1, pp. 107 - 126.

Matazzoni, J., 1991. Affordable Color Scanning. Macworld, June, pp. 130 - 137.Meier, H.R., 1984. Progress by Forward Motion Compensation in Cameras. Proc. of 15th IS-

PRS Congress, Vol. 25/A1, pp. 194 - 203.Rollei, 1993. Rollei RS1-C. Product brochure, Rollei Fototechnic GmbH, PO Box 3245,

38022, Braunschweig, GermanySchroeder, M., 1992. Rauemliches Aufloesungsvermoegen einer digitalen CCD-Kamera. Proc.

of 17th ISPRS Congress, Vol. 29/B1, pp. 183 - 189.Seiter, C., 1993. Low-Cost Color Scanners. Macworld, November, pp. 128 - 135.Steiger, W.F., 1991. Farbige DTR-Scanner und EBV-Scanner im Qualitaetsvergleich. DruckIn-

dustrie, No. 20, pp. 19 - 25.Trinder, J., 1987. Measurements in Digitised Hardcopy Images. Photog. Eng. and Rem. Sens.,

Vol. 53, No. 3, pp. 315 - 321.Trinder, J., 1989. Precision of Digital Target Location. Photog. Eng. and Rem. Sens., Vol. 55,

No. 6, pp. 883 - 886.UGRA/FOGRA, 1998. Brochure on Reproduction Test Chart 1988. UGRA, c/o EMPA, Unter-

str. 11, Postfach 977, 9001, St. Gallen, Switzerland, fax +41-71-227220.Wilkins, D., 1990. Digital Photogrammetric Applications with the Prime-Wild S9 Analytical

Plotter. Proc. of ISPRS Com. II Symp., Vol. 28/2, pp. 35 - 42.

3-line color CCD from Dalsa (3 x 2098 contiguous pixels, 3 x 8-bit, 4kHz line rate, 30MHz sensor data rate, 8 lines separation between the 3 lines, balance of each colour sensitivityindependently for a given scanning speed and lighting conditions to achieve maximumdynamic range for all colours). 4096 pels, 7 µm, 60 MHz, 10 µm, 120 MHz sensor data rate,6000 pels, 10 µm 60 MHz, Technolink (Americas), Cameras 10,000 pels, & MHz, 12,670 $,5000, 20 MHz, 4775$, 5000 , 10 MHz, 3077$, 7500 10 MHz, 8334 $, Thomson 5184, 7 µm,4000:1, maximum data rate 20 MHz, 4250 $

TDI technology for imaging in low light or high speed scanning conditions. Muchimproved sensitivity (noise floor) than comparable linear CCDs, 2048 pels, 13 µm, 120 MHzsensor data rate, 96 TDI stages, 6000 pels, 10 µm.

EG&G Reticon (all with antiblooming) 2048 13 µm, 13,000:1, 20 MHz data rate, TDI2048 x 32,64,96, 13. µm, 1300:1 120 MHz or 2048 x 64, 27 µm 5500:1, 128 MHz

IDark Signal Non-Uniformity, Photo Response Non-Uniformity, MTF or CTF, QuantumEfficiency (or sensitivity), charge transfer efficiency, dark current noise, smear, blooming,dynamic range, linearity, saturation level, noise floor or Noise Equivalent Power, spectralresponsivity, SNR, grey level resolution, maximum charge storage capacity, video data rate.

Joyce-Loebl 2605 drum scanner (12.5-1000 µm, maximum 1000 x 1200 mm, 1-10rev/s,refl and transm., 0-2D or 0-4D, Optronics 5040 drum scanner/laser plotter (12.5 - 200 µm, upto 1000 rev/sec maximum 1270 x 1016 mm only for reflective)

a case study with agfa horizon evaluation of dtp scanners · 2020. 3. 26. · evaluation of dtp...

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